In nuclear reactor vessels there is located a thermal shield which effectively separates or delineates the core of the nuclear reactor from the remainder of the vessel. The function of this thermal shield is as its name implies, i.e. to thermally shield the core from the outer walls of the reactor. For proper functioning, the thermal shield must be physically restrained within the vessel, but it must be restrained in such a manner that the shield can expand and contract as needed in accordance with the heat output of the core. Thus the function of the spacer pins.
Generally, spacer pins are threaded through openings in the shield from the inside during the assembly of the reactor vessel internals. Each pin is screwed through its respective opening until contact is made with a spacer pad secured to the reactor vessel wall. After the numerous spacer pins are thusly installed and aligned (there usually being at least two levels or elevations of such pins), they are each backed off the pad about one sixteenth of an inch (1/16") before they are welded to the thermal shield. This will secure the pin in place and prevent it from coming loose or unscrewing. Afterwards, the inner tail of the pin is removed and then ground down so that the spacer pin will be flush with the inside surface of the thermal shield.
As can be seen, such assembly requires a considerable number of operations which only increases the cost and the amount of labor required for each pin. Additionally, once the pin is welded in place, it cannot easily be removed or adjusted if needed.
It is thus an object of this invention to provide a spacer pin that mechanically locks to the thermal shield so as to avoid the need for the pin to be welded in place. Another object of this invention is to provide a spacer pin that can be easily removed should such be desired. A further object of this invention is to provide a spacer pin that is adjustable to allow for the setting and resetting of the gap between the thermal shield and the reactor vessel if it becomes necessary to do so. Still another object of this invention is to provide a spacer pin that can be installed flush to the inside surface of the thermal shield so as to eliminate the time-consuming task of machining or grinding the pin flush as previously required. Another object of this invention is to provide a spacer pin that is reusable and one that has minimal void area in order to reduce neutron streaming. These and other objects and advantages will become apparent upon further investigation.